1. Field of the Invention
This invention relates to an apparatus for controlling an output of an AC generator for a vehicle of the type which is adapted to control the output voltage of the AC generator in accordance with the level of an electric signal supplied from an external control unit.
2. Description of the Related Art
FIG. 18 is a diagram which illustrates the construction of a conventional apparatus for controlling an output of an AC generator for a vehicle as disclosed in, for example, Japanese Patent Laid-Open No. 62-107643. An AC generator 1 driven by an engine (not shown) comprises an armature coil 101 and a field coil 102. A rectifier 2 rectifies all waves of the AC output from the AC generator 1 to transmit the rectified output through a positive terminal 201 and a negative terminal 202 thereof. A voltage regulator 3-1 controls the rectified output from the AC generator 1 to a predetermined value. An external control unit 4 receives signals indicating the state of operation of a vehicle in the form of an automobile from a variety of sensors SE1 to SE4 attached thereto to transmit an output instruction signal to the voltage regulator 3-1.
A battery 5 is electrically charged with an output from the AC generator 1 through the rectifier 2. An electric load 6 of the automobile is supplied with electric power from the battery 5 through an electric load switch 7. Upon closing of a key switch 8, voltage is supplied through the key switch 8 from the battery 5 to a constant-voltage circuit VS in the voltage regulator 3-1. A stabilized voltage output by the constant-voltage circuit VS is used as a comparative reference voltage (hereinafter called "reference voltage") VC for the voltage of the battery 5 to be described later. The voltage regulator 3-1 includes: a series circuit comprising voltage dividing resistors R3a and R3b connected in series between the positive terminal of the battery 5 and the ground; a series circuit comprising a resistor R1 and a transistor Q4 connected in parallel to the voltage dividing resistor R3b, an end of which is connected to the ground; a series circuit comprising a resistor R2 and a transistor Q1; and a capacitor CA. The voltage regulator 3-1 further includes a comparator CP, whose positive input terminal receives a divided voltage VA Of a generator output voltage VG appearing at a connection point between the voltage dividing resistors R3a and R3b and whose negative input terminal receives the reference voltage VC supplied from the constant-voltage circuit VS to generate a high- or low-level comparison signal through a resistor R4 connected to the output terminal thereof.
It is to be noted that the base of each of the transistors Q1 and Q4 is supplied from the external control unit 4 with a logic signal, the level of which corresponds to the state of the operation of the automobile.
The voltage regulator 3-1 further includes: an RC series circuit comprising a resistor R5 connected in series between the positive terminal and the negative terminal of the rectifier 2 through the external connection terminal and a capacitor CA with a connection point connected to an output resistor of the comparator CP; a series circuit comprising a resistor R6 connected in series to the collector of a transistor Q2 whose base receives a comparison signal from the connection point of the RC series circuit; and a series circuit comprising a diode D1 connected in series to the collector of a transistor Q3, the base of which receives voltage developed between the collector resistor R6 and the base.
The diode D1 is connected in parallel to the field coil 102 through an external connection terminal to absorb a surge which is generated when the field coil 102 is turned off. Since the structure of the external control unit 4 is substantially unrelated to the present invention, the description of the operation of the external control unit 4 is omitted here.
The operation of the above-described conventional apparatus will now be described.
When a driver of the automobile turns on the key switch 8 to start the engine, the voltage of the battery 5 is supplied from the battery 5 to the constant-voltage circuit VS through an ignition terminal so that the reference voltage VC is generated. The reference voltage VC is supplied to the negative input terminal of the comparator CP for comparison with the divided voltage VA of the generator output voltage VG received by the positive input terminal.
When the divided voltage VA is lower than the reference voltage VC set in the constant-voltage circuit VS, the output level from the comparator CP is low before the low level signal is supplied to the base of the transistor Q2 so that the transistor Q2 is turned off. As a result, the ensuing transistor Q3 is turned on with a potential developed across the resistor R6 connected to the base of the transistor Q3.
When the transistor Q3 is turned on, a loop passing the positive terminal of the rectifier 2, the field coil 102, the transistor Q3 and ground is formed. Then, a field current flows from the battery 5 to the field coil 102 of the AC generator 1 to thereby start generation of electric power. When the rotational speed of the AC generator 1 has been increased due to the start of the engine rotation, the generator output voltage VG detected at the positive terminal of the rectifier 2 increases.
As a result, the divided voltage VA obtained by dividing the generator output voltage VG with the voltage dividing resistors R3a and R3b increases above the reference voltage VC. Therefore, the level of the output from the comparator CP becomes high and the high-level output is received by the base of the transistor Q2 so that the transistor Q2 is turned on. When the transistor Q2 is turned on, the transistor Q3 is turned off to break up the field current loop, thus reducing the field current and hence the voltage VG generated by the AC generator 1.
When reduction in the generator output voltage VG below the reference voltage VC has been detected from the output of the comparator CP, the transistor Q2 is again turned off and the transistor Q3 is again turned on. Thus, the field current starts flowing. By repeated turning on and off of the field current as described above, the generator output voltage VG can be controlled to a predetermined level. Thus, the voltage of the battery 5 charged by the AC generator 1 can be controlled to a substantially constant level.
However, when the AC generator 1 is operated by the engine, the output of the generator output power must be controlled in accordance with the state of operation or driving of the automobile in order to reduce the engine load. Accordingly, the generator output voltage is switched over into three levels in accordance with the engine load and the running speed of the automobile so as to control the output power generated by the AC generator 1. The three levels includes a low level in which the generator output voltage is lower than a first predetermined value, a high level in which the generator output voltage is higher than a second predetermined value larger than the first predetermined value, and an intermediate or usual level in which the generator output voltage is equal to or between the first and second predetermined level.
If the generator output is set to the low level which is lower than the usual or intermediate level, the external control unit 4 supplies signals for turning the transistors Q1 and Q4 off to the bases thereof. Thus, the divided voltage VA is determined in accordance with only the ratio of the resistance values of the voltage dividing resistors R3a and R3b, as given by the following equation: EQU VA=VG(R3a/(R3a+R3b))
Thus, the output power of the AC generator 1 is set to the level lower than the usual level.
If the output power of the AC generator 1 is set to the usual level, the external control unit 4 supplies a signal for turning the transistor Q4 on to the base thereof and a signal for turning the transistor Q1 off to the base thereof. As a result, the resistor R2 is connected in parallel to the voltage dividing resistor R3b, so the divided voltage VA is given by the following equation: EQU VA=VG((R2.multidot.R3b)/(R2.multidot.R3b+R3a))
where R2.multidot.R3b is a parallel resistance value of the resistor R2 and the resistor R3b. Thus, the generator output power is set to the usual level.
If the output power of the AC generator 1 is set to the level higher than the usual or intermediate level, the external control unit 4 supplies a signal for turning the transistor Q4 on to the base thereof and a signal for turning the transistor Q1 on to the base thereof. As a result, the resistors R1 and R2 are connected in parallel to the voltage dividing resistor R3b, thus providing the divided voltage VA as follows: EQU VA=VG((R1.multidot.R2.multidot.R3b)/(R1.multidot.R2.multidot.R3b+R3a))
where R1.multidot.R2.multidot.R3b is a parallel resistance value of the resistors R1, R2 and R3b. Accordingly, the generator output power is set to the level higher than the usual or intermediate level.
Therefore, the level of the divided voltage VA is made to be any of the three levels in accordance with the logic (whether the level is "High" or "Low") of the signals respectively supplied to the bases of the transistors Q1 and Q4. Thus, the generator output power can be adjusted to any of the three levels.
In the conventional apparatus, the generator output power is controlled to adjust the voltage for electrically charging the battery 5 in such a manner that a resistor is connected in parallel to any of the plurality of resistors connected in series so as to change the divided voltage to thereby alter the level of the generator output voltage.
However, another apparatus of a simpler circuit arrangement may be available in which one of the voltage dividing resistors is short-circuited by turning on an associated transistor to change the divided voltage, thus adjusting the generator output power.
FIG. 19 is a diagram which illustrates the arrangement of such an apparatus which is known to the inventors of the present invention. Referring to FIG. 19, the like reference numerals as those shown in FIG. 18 represent the same or corresponding elements. A voltage regulator 3-2 comprises: resistors 301 to 303 connected in series between the positive terminal of the battery 5 and ground so as to serve as voltage dividing resistors; and a transistor Q1a having a collector and an emitter respectively connected to the opposite ends of the resistor 303 with one end thereof grounded and a base which receives a high-level or low-level signal from the external control unit 4. The voltage regulator 3-2 further comprises: a Zener diode ZD1, the cathode of which is connected to the connection point between the voltage dividing resistors 301 and 302 and which is made electrically conductive when the divided voltage VA increases above a breakdown voltage level thereof; and a transistor Q2, the base of which is connected to the anode of the Zener diode ZD1, the emitter of which is grounded and the collector of which is connected to the output terminal of the key switch 8 through the resistor 304.
The base of the transistor Q1a, which constitutes part of the voltage regulator 3-2, is connected through a resistor 300 to the output terminal of the key switch 8, whereas the collector of the transistor Q2 is connected to the base of the transistor Q3. When the key switch 8 is switched on upon start of the engine, a current is supplied from the battery 5 to the transistors Q1a and Q3 so that the transistors Q1a and Q3 are thereby turned on.
The operation of this apparatus will now be described. When the key switch 8 is closed upon engine starting, an electric current flows from the battery 5 to the base of the transistor Q3 through the resistor 304 of the voltage regulator 3-2. Thus, the transistor Q3 is turned on, allowing a field electric current to flow from the battery 5 to the field coil 102. As a result, the AC generator 1 is brought to a state where the AC generator 1 is able to generate electric power.
When the engine is started and thus the AC generator starts generating the electric power, the voltage at the positive output terminal 201 of the rectifier 2 is raised. Thus, the battery 5 is electrically charged to increase its voltage. A transistor 401 of the external control unit 4 usually generates a high-level signal, so the transistor Q1a of the voltage regulator 3-2 is brought to a conductive state and the voltage dividing resistor 303 is short-circuited.
The voltage across the positive and negative terminals of the battery 5 has been detected on the basis of the divided voltage VA generated by the voltage dividing resistors 301 and 302. When the battery voltage has been raised to increase the divided voltage provided by the voltage dividing resistors 301 and 302 above the breakdown voltage for the Zener diode ZD1, the Zener diode ZD1 is rendered electrically conductive to allow a current to flow into the base of the transistor Q2, thus turning it on.
When the voltage of the battery 5 has become lower than a predetermined level to reduce the divided voltage below the breakdown voltage, the Zener diode ZD1 is rendered non-conductive and the transistor Q2a is turned off. Thus, the transistor Q3 is turned on or off in accordance with whether the transistor Q2 is turned on or off. As a result, the field electric current flowing through the field coil 102 is repeatedly turned on and off so that the output power of the AC generator 1 is controlled to the normal level. As a result, the voltage to be charged to the battery 5 can be set to the usual level.
When the transistor 401 with the transistor Q1a being turned on generates a low-level signal in response to an signal input from any of a variety of sensors for detecting the state of driving or operation of the automobile, the transistor Q1 I of the voltage regulator 3-2 is turned off and thus the voltage dividing resistor 303 is rendered into serial connection to the voltage dividing resistor 302. As a result, the voltage of the battery 5 is detected by the voltage dividing resistors 302, 303 and 304. Since the voltage of the battery 5 is set to the level lower than the usual level, the voltage to be charged to the battery 5 is set to the level lower than the usual level.
The apparatus shown in FIG. 19 is constructed such that the voltage of the battery 5 is supplied from its positive terminal to an end of the serially connected voltage dividing resistors through a lead wire so as to detect the charged voltage of the battery. However, if the lead wire is disconnected due to an accident or the like, charging of the battery 5 would sometimes be uncontrollable, causing over- charging.
An apparatus equipped with a circuit for preventing over-charging of a battery due to such an accidental disconnection of a lead wire is also known to the inventors of the subject application.
FIG. 20 is a diagram illustrating the arrangement of such an apparatus. In this figure, in addition to the components shown in FIG. 19, a voltage regulator 3-3 of this apparatus includes a circuit for preventing over-charging of a battery which comprises: voltage dividing resistors 305 and 306 series connected in series between the positive terminal of the battery 5 and ground through the key switch 8 and a indicator lamp 9 series connected thereto; and a diode D3 having an anode thereof connected to a connection point between the voltage dividing resistors 305 and 306 and a cathode thereof connected to the cathode of the Zener diode ZD1. The resistance values of the voltage dividing resistors 305 and 306 are set such that when the voltage to be charged to the battery 5 has been raised to about 15.6 V, the breakdown voltage for the Zener diode ZD1 is developed at the connection point between the voltage dividing resistors 305 and 306 through the diode D3. A rectifier 2A has a sub-terminal 202 of a rectifier for supplying an field electric current to the field coil 102 at the time of an initial power generation stage of the AC generator 1.
The operation of the apparatus shown in FIG. 20 will now be described. When the key switch 8 is turned on, an electric current flows from the battery 5 to the base of the transistor Q3 through the resistor 304 so that the transistor Q3 is thereby turned on. As a result, the field electric current is supplied to the field coil 102 to thereby turn on the indicator lamp 9.
When the engine has been started and the AC generator 1 starts generating electric power, the voltage at the sub-terminal 203 increases to substantially the same level as the voltage of the battery 5, thus turning off the indicator lamp 9. At this time, the charging electric current flows from the positive terminal 201 of the rectifier 201 to the battery 5 so that the battery 5 is electrically charged. Furthermore, electric power is also supplied from the rectifier 201 to the load 6 through the load switch 7.
Since the transistor 401 of the external control unit 4 is in a non-conductive state, the transistor Q1 is in a conductive state because an electric current is being supplied to the base of the transistor Q1 from the sub-terminal 203 through the resistor 300. Therefore, the divided voltage VA for the battery voltage is determined on the basis of the resistance ratio of the resistors 301 and 302.
When the divided voltage VA has been determined as described above, an offset voltage, which is determined on the basis of the divided voltage VA, is supplied to the cathode of the Zener diode ZD1 through the diode D3. As the AC generator 1 starts power generation, the charging voltage or output voltage of the AC generator 1 is increasing to raise the potential of the voltage detection terminal (i.e., positive terminal) of the battery 5 to, for example, about 14.4 V as shown in FIG. 21. At the same time, the divided voltage VA is increased to reach the breakdown voltage for the Zener diode ZD1.
As a result, the Zener diode ZD is rendered electrically conductive, turning on the transistor Q2 and turning off the ensuing transistor Q3. Thus, the field electric current supplied to the field coil 102 is interrupted. The interruption of the field electric current stops power generation of the AC generator 1, thus preventing over-charging of the battery 5. Thereafter, when the voltage of the battery 5 decreases below a predetermined level such as 14.4 V, the transistor Q2 is turned off and the transistor Q3 is turned on, whereby the AC generator 1 resumes power generation again to start charging of the battery 5.
In a case where no electric load for the battery 5 is required and the voltage of the AC generator 1 is intended to be adjusted to about 12.8 V, the transistor 401 of the external control unit 4 is rendered conductive to turn the transistor Q1 off. As a result, the voltage dividing resistor 303 is connected in series to the voltage dividing resistor 302 so that the voltage dividing ratio of the battery voltage is increased. Accordingly, the offset voltage with respect to the Zener diode ZD1 is raised, thus increasing the output power of the AC generator 1. When the voltage to be charged to the battery 5 is thus raised to about 12.8 V, the Zener diode ZD1 is rendered electrically conductive. As a result, the transistor Q2 is turned on and hence the ensuing transistor Q3 is turned off to interrupt the field current, so the AC generator 1 ceases power generation, thus adjusting the generator output power to 12.8 V. By repeatedly turning the output of the transistor 401 of the external control unit 4 on and off as described above, the voltage to be charged to the battery 5 can be adjusted to 12.8 V or 14.4 V as shown in FIG. 21.
The foregoing apparatuses constructed as described above have the following disadvantages.
First, with the first-mentioned apparatus shown in FIG. 18, both the voltage regulator 3-1 and the rectifier 201 are generally incorporated in or integrally formed with the AC generator 1 due to the necessity of eliminating the influence of a voltage drop in the grounding circuit on the adjustment voltage for the AC generator 1. Therefore, two lead lines are required which extend from the respective bases of the transistors Q1 and Q4 in the voltage regulator 3-1 for connection with the external control unit 4 in order to supply control signals from the external control unit 4 to the voltage regulator 3-1. Accordingly, two terminals for connection of the two lead lines are required as well. Thus, reliability in connection of the two lead lines to the terminals is required, complicating wiring operation and hence increasing the overall cost of manufacture of the apparatus.
Another problem arises with the second-mentioned apparatus shown in FIG. 19 in that the generator output voltage can be adjusted to only two levels consisting of a usual level and a low level lower than the usual level in dependence on whether the level of the control signal supplied from the external control unit 4 to the voltage regulator 3-2 is high or low and, therefore, the output voltage of the AC generator 1 cannot be adjusted finely so as to correspond exactly to the state of driving or operation of the automobile.
Moreover, with the last-mentioned apparatus shown in FIG. 20, the adjustment or target voltage for the AC generator 1 can be switched over from a usual level of 14.4 V to a low level of 12.8 V when no electric load is required or connected, so that the load of the AC generator 1 acting on the engine is reduced, thus improving fuel consumption. In this case, however, the rapid switching of the adjustment or target voltage from the usual level of 14.4 V to the low level of 12.8 V abruptly reduces the output voltage of the AC generator 1. Due to the resultant abrupt reduction in the load of the AC generator 1 acting on the engine, there arises a further problem in that the rotational speed of the engine increases abruptly.
On the other hand, when the adjustment or target voltage is restored from 12.8 V to 14.4 V to meet the requirement for increased power generation, the adjustment or target voltage is rapidly switched over as well, so that the engine torque required for driving the AC generator 1 is also rapidly increased. Thus, the load of the AC generator 1 acting on the engine abruptly increases accordingly, giving rise to a problem that the rotational speed of the engine is abruptly reduced. This results in unstable operation of the engine.